1. Field of the Invention
The present invention relates to a multi-layer substrate including a waveguide for transmitting a radio frequency signal and a method of manufacturing the multi-layer substrate.
2. Description of the Prior Art
A wireless communication technology, in which a Radio Frequency (RF) signal such as a millimeter wave is used, is employed for data communication, an automobile radar, etc.
The RF signal, which is used in wireless communication, should be ensured to have a good transmission property in a Printed Circuit Board (PCB), etc.
Meanwhile, the automobile radar may include an RF signal processing unit for processing an RF signal using an antenna for transmitting/receiving an RF signal having a high frequency and converting the RF signal into an Intermediate Frequency (IF) signal or a baseband signal; and a baseband signal processing unit for processing the IF signal or the baseband signal. These circuit components can be configured by one or more chips and can be mounted on the PCB, etc.
At this time, a predetermined feed structure may be used to transmit the RF signal having a high frequency to the RF signal processing unit, and an example of the feed structure is a micro-strip line or a waveguide.
That is, when a circuit, in which an RF is used, is designed, techniques such as an RF micro-strip line, a waveguide, etc. may be used in order to reduce loss of a moving path of an RF signal.
In particular, when both an antenna pattern and an RF processing circuit (RF circuit) are arranged on one surface of the PCB, a metal pattern such as a micro-strip may be configured to transfer a signal between the antenna pattern and the RF circuit.
However, when the antenna pattern and the RF circuit are arranged on different surfaces of the PCB, a signal transmission structure, which passes through a substrate, should be formed to transmit an RF signal between the antenna pattern and the RF circuit. A structure of the waveguide may be used to transmit such an RF signal, and a waveguide, which is formed on a multi-layer wiring board, may be called a laminated waveguide.
When the micro-strip or the laminated waveguide is formed to have a high area density, the transmission path of an RF signal may be changed from the plane direction to the thickness direction of a substrate.
That is, an RF signal, which has moved in the plane direction of a substrate, may move in a direction vertical to the substrate along the laminated waveguide formed in the thickness direction of the substrate through the micro-strip pattern formed on the surface of the substrate.
In this way, when the transmission path of an RF signal is changed to the thickness direction of the substrate by the laminated waveguide, the RF signal is reflected at a portion where the direction is changed, so that a transmission loss is increased, and thus, the signal transmission property of the laminated waveguide may deteriorate.
In particular, as the RF signal has a frequency of tens of GHz, an effort to reduce the transmission loss caused by the transmission of the RF signal through the waveguide is required. To this end, the structure of the waveguide should be precisely formed.
That is, the waveguide in the substrate, which is used as a transmission structure of the RF signal corresponding to tens of GHz, should be manufactured with excellent precision in an assembling or forming process, and a performance difference resulting from an assembling tolerance or a forming tolerance greatly influences a performance of a radar, etc.
In particular, since a plurality of circuit elements should be mounted in signal transmission/reception apparatuses such as a recent automobile radar, etc., the substrate has a multi-layer structure having two or more layers, and thus, when a waveguide is formed in the multi-layer substrate, an assembling tolerance or a forming tolerance of the waveguide becomes important.